Cause Puccinia pelargonii-zonalis, an autoecious (can complete their life cycle on this one host) rust fungus that survives on living plants and plant debris. Movement of infected cuttings can also spread this fungus. Infections occur readily if leaves are wet 5 to 6 hours at 55°F to 75°F. Urediniospores may be infective at least 3 months. Considerable genetic variation exists from highly susceptible to immune cultivars.
Symptoms Both leaf surfaces first develop white or yellowish spots. Spots on the lower surface enlarge and become blister-like pustules. These brown powdery pustules (uredinia) occur on leaves and, less frequently, on petioles and stems. Severe leaf infection causes yellowing and defoliation. Uredinia containing spores may be in concentric rings and may be surrounded by yellow tissue.
- Destroy diseased leaves and severely affected plants. Place into a closed container for removal from the greenhouse.
- Water at base of plants and/or with drip irrigation. Do not wet foliage.
- Reduce humidity.
- Space plants to provide good air circulation.
- Inspect any new plants and all stock plants for signs of the disease.
Chemical control Combined with regular scouting, use fungicides when rust first appears and apply at two-week intervals using two (2) to four (4) sprays a season.
- Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
- Bicarbonate-based products. Do not mix with acidifying agents. Thorough coverage is essential. O
- MilStop (85% potassium bicarbonate) at 1.25 to 5 lb/100 gal water. Oregon and Washington only. 1-hr reentry.
- Monterey Bi-Carb Old Fashioned Fungicide at 4 teaspoons/2 gal water H
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. H
- Broadform at 4 to 8 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Compass 50 WDG at 2 to 4 oz/100 gal water. Do not use organosilicate additives. Group 11 fungicide. 12-hr reentry.
- Daconil Weather Stik at 1.4 pints/100 gal water. Group M5 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Has good eradicant activity. Group 11 fungicide. 4-hr reentry.
- Insignia SC at 3 to 6 fl oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
- Mancozeb-based products. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.
- Pageant at 6 to 12 oz/100 gal water. Do not use with organosilicone-based adjuvants. Must be used before infection. Group 7 + 11 fungicide. 12-hr reentry.
- Phyton 27 at 1.5 to 4 fl oz/10 gal water. Group M1 fungicide. 48-hr reentry.
- ProStar 70 WG at 3 to 6 oz/100 gal water. Group 7 fungicide. 12-hr reentry.
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 1 fl oz/gal water. Group 3 fungicide. H
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
- Terraguard SC at 2 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Tourney 50 WDG at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Trigo at 4.8 to 9 oz/100 gal water. Group 3 + 11 fungicide. 12-hr reentry.
Reference Harwood, C.A. and Raabe, R.D. 1979. The disease cycle and control of geranium rust. Phytopathology 69:923-927.